The present invention is in the field of electrical measurement systems, and more particularly relates to systems for measuring current.
There are many prior art systems for measuring electrical current that are operative from d.c. to relatively high frequencies. One such approach uses a small resistive shunt coupled to a high gain differential amplifier. However, with this approach, to minimize power dissipation, shunts are generally required to be small and consequently suffer from relatively low sensitivity. Moreover, shunts do not provide electrical isolation or common mode rejection.
Another approach incorporates flux nulling in a toroidal magnetic core positioned about the conductor carrying the current to be measured. With this approach, a Hall effect device senses flux in a gap in the core, and in response drives appropriate current in a bucking winding on the core to null the flux induced by the current-being-measured. Within the bandwidth of this nulling feedback loop, the easily measured bucking winding current corresponds to the current being measured. However, the sensitivity of such Hall effect current sensors is very limited due to the fact that the toroidal core must contain an air gap for the Hall effect device. Such devices are also highly sensitive to external magnetic fields and their offset voltage drifts, leading to errors in measurement. Moreover, the circuitry necessary to drive the bucking winding and close the loop must be high gain, low noise, and very stable in order to sense currents in the milliampere region.
Yet other prior art flux nulling techniques are known. Several of these techniques incorporate a core having a square B-H characteristic which is adapted to oscillate as a flux oscillator. In such systems, however, a substantial voltage is generally induced on the input and bucking windings which has substantial effects on the accuracy and stability of the nulling system. Moreover, in general, the prior art flux nulling approaches are characterized by relatively limited frequency ranges of operation.
It is an object of the present invention to provide an improved current sensor.
It is yet another object to provide an improved current sensor operable from d.c. to relatively high frequencies characterized by high accuracy and relatively small offset drift.
Yet another object is to provide an improved current sensor having a relatively large dynamic range.